The present invention relates to control of laser waveform upon reproduction of information in an optical disk apparatus.
In an optical disk apparatus using a laser beam emitted from a laser diode to read information of mark and space recorded in an optical disk, it is important to decide binary information of mark and space without error. However, there is a problem that noise such as mode hop noise and return light noise is produced in the laser beam.
As a method of reducing such noise, JP-A-2000-149302 discloses a method named high-frequency superposition. Further, as a method of controlling the laser power upon reproduction, JP-A-2005-216395 discloses a method named automatic power control (APC).
Recently, optical disks having different standards such as CD, DVD, Blu-Ray disc (hereinafter abbreviated to BD) and HD-DVD are taken to the market. These optical disks are not compatible with one another, although optical disk apparatuses dealing with recording and reproduction for CD, DVD and BD and optical disk apparatuses dealing with recording and reproduction for CD, DVD and HD-DVD are taken to the market in consideration of user's convenience. However, an optical disk apparatus dealing with recording and reproduction of both of BD and HD-DVD is not taken to the market yet.
It is considered as a cause of the reason that there are different points on the standards between the optical disks conforming to the BD standards (hereinafter referred to as “optical disks 1”) and the optical disks conforming to the HD-DVD standards (hereinafter referred to as “optical disks 2”) as shown in FIG. 4. The relation of the wavelength of the laser beam (405 nm for both of BD and HD-DVD) and the numerical aperture of lens (0.85 for BD and 0.65 for HD-DVD) prescribed in the standards is considered as being particularly a problem among them.
When the wavelength of the laser beam is λ and the numerical aperture of lens is NA, it is known that a spot diameter of the laser beam on the optical disk is proportional to:(λ/NA)  (expression 1)and a spot area on the optical disk of the laser beam is proportional to:(λ/NA)2  (expression 2)That is, the spot areas on the optical disk are different between the BD standards and the HD-DVD standards. The reproduction laser power values for the BD standards and the HD-DVD standards are made different to be 0.3 mW and 0.5 mW, respectively, so that the power of the laser beam per unit area is made substantially equal.
When the characteristics (amplitude and frequency) of the high-frequency signal superposed in order to reduce noise of the laser beam are fixed, satisfactory reproduction can be made for one optical disks, although there arises a problem that satisfactory reproduction cannot be made for the other optical disks and more particularly there arises a problem that data recorded on the optical disk is destroyed.